NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Oct 26, 6:21 PM EST, 2 hours 29 minutes earlier.
Area forecast discussion, Oct 26, 6:21 PM EST, 2 hours 29 minutes before
Area Forecast Discussion
National Weather Service Boston/Norton MA
721 PM EDT Tue Oct 26 2021
.SYNOPSIS...
Strong coastal storm passing offshore will bring the potential
for heavy rain, flooding, and damaging winds tonight and
Wednesday. Much quieter and mainly dry weather returns Thursday
into at least part of Friday. A frontal system brings periods of
rain, heavy at times, Friday night into Saturday night.
Improving conditions with dry weather Sunday into the first
couple days in November, with seasonable temperatures.
&&
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
700 PM Update:
Conditions are beginning to deteriorate especially across
eastern MA and portions of RI as deepening coastal low near
about 41N/69W continues to pivot towards the WNW. As of this
writing, buoy 44011 measured a sea-level pressure of 986 mb
which to this point is the lowest seen so far.
Cancelled the Flood Watch from the remaining portion of the
eastern slopes of the Berkshires. Based on observed rain, river
levels and expected additional rain of 0.5 to 0.75 inches, the
Flood Watch for those areas was becoming increasingly difficult
to justify. Hence, the cancellation. KBOX radar showing the most
intense rain bands now approaching the Outer Cape. Rain will
turn heavy at times tonight into a good part of Wednesday
morning across remaining portions of the Flood Watch area, but
especially eastern MA and RI. Given strong to damaging winds
that may lead to tree or powerline damage onto roads, in
addition to urban and poor drainage flood issues, it could make
for a very difficult Wednesday morning commute.
Wind gusts to this point have been on the lower end of
expectations so far, though some gusts in the 40 to 45 mph range
have been observed in southeast MA. With the approach of the
cyclone, the KBOX radar now shows a fairly well-sampled 55 kt
jet at 3000 ft coming into South Shore, and is stronger just
offshore. As this jet and the low approaches from the west,
northeast wind gusts will continue to increase overnight and
remain strong into Wednesday morning. Thus there remains no
change to the thinking regarding winds, both in magnitude and
area.
Overall message is that conditions will continue to deteriorate
over the next several hours.
Previous discussion:
Low pressure centered just outside of 40N/70W this evening.
Consensus is that this system turns west early tonight, moving
across Nanucket Sound overnight and then turning south by morning.
Low level flow north of the center and moving west will come ashore
and run into a low level ridge, generating low level lift and
resulting rainfall. PW around 1.25 to 1.4 inches is well above
average, so expect periods of locally heavy rain during the night.
Heaviest rain should be in Eastern MA, lesser amounts in the CT
Valley. Range of rainfall tonight into Wednesday will be 0.5 inches
to 2 inches. Flood Watch continues overnight with poor drainage
flooding most likely.
Strong winds aloft with 75-85 kt at 850-mb to 950-mb. This suggests
potential for 50-60 kt/60-70 mph winds at the coast, with lesser
values farther west. High Wind Warning continues in Eastern MA and
Srn RI tonight, with a Wind Advisory in the rest of RI, NE CT, and
Central MA.
&&
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Overview...
Offshore storm turns east during Wednesday and moves out to sea.
High pressure builds into New England later Wednesday night.
Details...
Surface low and closed upper low turn east and move out to sea.
Northeast surface flow continues Wednesday and Wednesday evening.
Rain tapers off from west to east during the day, but lingers in
Eastern MA through at least part of Wednesday night.
Winds at low levels will diminish a little, but still a decent wind
with speeds of 50-60 kt, producing surface gusts of roughly 35 mph
in the evening. These winds will diminish further Wednesday night,
but still gusty northeast winds along the coast.
The northeast wind will maintain an onshore flow. Thus expect rain
in Eastern/Central areas Wednesday, shrinking to pockets of rain
Wednesday night.
&&
.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights:
* Seasonable Thurs, improving conditions for coastal New England.
Leads to brief dry weather thru Fri.
* Frontal system brings another period of steady rains Fri night
into Sat night. Rain may be heavy at times and could lead to
flooding given clogged storm drains and already wet conditions.
* Dry and seasonable Halloween into the first couple days in
November.
Details:
Thursday:
Still dealing with lingering effects across Cape Cod and the eastern
waters from the strong autumn nor'easter, but the peak of the
impacts will have already passed. Ocean-effect type rain showers
(little if any additional rainfall accumulation) may linger, along
with still fairly brisk northerly gusts early Thursday across the
Cape. Continued rough seas across the eastern waters. However sfc
ridge from a 1024 mb high pressure over northern Quebec will
gradually build southward through Southern New England. Overall
comparatively better conditions inland, with gradually improving
conditions across Cape Cod and the Islands as Thursday progresses.
Thursday Night through Friday:
Ridging both at surface and aloft to continue into Thurs nite. This
will support a period of clear skies and light winds across the
interior, favoring strong evening longwave radiational cooling
effects. The more dicey element to the forecast is the sky cover, as
some models show some leftover RH returning back landward, mainly
for the south coast. It does look like eastern/coastal areas stand
to be the mildest (upper 40s) given continued though light NE to ENE
winds. Meanwhile stronger cooling in the interior should favor lows
mid/upper 30s.
On Friday, on the synoptic level, ridging initally in place weakens
as we await the next system moving in from the central Appalachians.
Vertically-stacked cyclone initially over eastern TN/eastern KY
early Fri moves very slowly northeast. This feature will translate
its sfc warm front or frontal occlusion northeast across the mid-
Atlantic states Fri and potentially into our southwestern areas by
very late Fri or Fri nite. Pressure gradient tightens on Friday,
supporting some enhancement to easterly onshore winds (daytime gusts
20-25 mph). On PoP, some variance in guidance on the timing, and
model RH profiles indicate some drier air to be overcome at onset as
well. Kept most of Fri dry, but then ramp up PoP toward chance east
to high likely west around and after sundown. Increasing clouds and
only modest warm advection supports 50s for highs in the interior,
upper 50s to low 60s south coast/Cape Cod and Islands.
Friday Night through Saturday Night:
Next weather system begins to affect our area in this period.
Vertically- stacked low near southwest VA early Fri nite moves
slowly NE toward the northern Adirondacks vicinity by 12z
Sunday.
Wettest period looks to be Friday night into Saturday, as warm front
or frontal occlusion brings rain to our area. Rather strong speed
convergence at 925-850 mb and PWATs increasing to around 1 to 1.25"
supports moderate to at times heavy rain intensity Friday night into
Saturday morning with the front. Potential for urban and poor
drainage type flooding can't be discounted, as well as nuisance
flooding associated with clogged storm drains from leaves expected
to fall from the storm tonight into Wednesday. The other
element that could also enhance rain rates on Saturday is
secondary low development south of LI Sat; this is indicated in
most of the 12z guidance but its position and ultimate track
varies. Have opted for Categorical PoP Fri night into much of
Sat. By Sat night, appears we get into the dryslot with fairly
rapid decrease in PoP down to Chance levels, with cool advection
also occurring on SW winds. First cut at rain totals is around
1.5-1.75", locally more possible.
Rain looks to be the greater impact at least at this point in time.
Winds at the moment don't appear to be impressive, and any gusts
likely should weaken Fri night as rain arrives. Did check on the
astro tides as well for late in the week which are still around 9 ft
for Boston Harbor. Given waves building to around 10 ft with modest
onshore winds, not thinking coastal flooding/splashover or beach
erosion is a concern as yet either.
Highs Sat in the upper 50s to mid 60s, lows mainly 40s to lower 50s
Fri and Sat nights.
Sunday/Halloween:
Closed low aloft deamplifies into an open wave trough which moves
across Southern New England. Fairly unsettled given cyclonic flow
aloft, with variable amounts of cloud cover and perhaps a spotty
light shower but dry most of the time. Under west winds, highs on
Halloween should reach into the low to mid 60s, with seasonable lows
in the 40s to around 50.
Early Next Week:
After a rather amplified pattern for much of this week into the
weekend, our weather pattern aloft transitions to one of broad zonal
flow with WSW mid-level winds aloft. Prominent northern-stream low
over Canada helps to bleed cooler air southward across the Midwest,
Great Lakes into far northern New England, but WSW flow aloft should
slow this cooler air and likely modify it as well. Followed NBM for
this period. This opts for dry weather while showing a gradual
cooldown in temperatures Mon/Tues but still just closer to
seasonable levels.
&&
.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
00z TAF Update:
Tonight: High confidence.
Widespread IFR/LIFR. Periods of rain, heavy at times, with vsbys 1-3
miles in rain and fog. Lowest visibilities in rain across
central and eastern airports. Increasing NE winds with gusts
25-35 kt in the CT Valley, 35-45 kt in the Central Hills, and
45-60 kt Eastern MA and coastal RI. Given the magnitude of
surface gusts being quite high, have opted to remove the mention
of low-level wind shear in TAFs but will be bumpy nonetheless.
Wednesday...High confidence.
Conditions improve to VFR in the west, but linger at IFR along
the Eastern MA Coast. Gusty NE winds 40-50 kt East Coast, and
20-30 kt in the CT Valley.
Wednesday night...High confidence.
VFR in western and central areas, MVFR cigs in RI/Eastern MA.
Northeast wind gusts 25-35 kt Cape Cod and Islands.
KBOS Terminal...High confidence in TAF. VFR early, lowering to IFR
in rain early tonight through Wednesday morning.
KBDL Terminal...High confidence in TAF. MVFR cigs/vsbys in rain.
Outlook /Thursday through Sunday/...
Thursday: Mainly VFR, with local MVFR possible. Windy with
local gusts up to 30 kt.
Thursday Night: Mainly VFR, with local MVFR possible. Breezy.
Friday: Mainly VFR, with areas MVFR possible. Windy with local
gusts up to 30 kt. Chance RA.
Friday Night: Mainly MVFR, with areas IFR possible. Windy with
local gusts up to 30 kt. RA.
Saturday: Mainly MVFR, with local IFR possible. Breezy. RA.
Saturday Night: Mainly VFR, with local IFR possible. Breezy.
Chance RA.
Sunday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.
&&
.MARINE...
Forecaster Confidence Levels.
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Storm Warnings continue for all waters tonight into Wednesday
morning. Winds and seas ramp up, with highest values tonight and
early Wednesday. Max wind gusts of 60-70 kt along the waters east of
Massachusetts. Dangerous seas developing, with highest values 20-25
ft along the waters east of Massachusetts. Vsbys will be reduced
to 1-3 mi in rain and fog.
Winds and seas begin to subside Wednesday night, with gusts
initially to 40 kt and seas up to 18 feet but diminishing.
Outlook /Thursday through Sunday/...
Thursday: Strong winds with gusts up to 30 kt. Rough seas up to
17 ft. Isolated rain showers.
Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 14 ft.
Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 11 ft. Chance of rain.
Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 10 ft. Rain.
Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 11 ft. Rain.
Saturday Night: Winds less than 25 kt. Rough seas up to 10 ft.
Chance of rain.
Sunday: Winds less than 25 kt. Rough seas up to 9 ft. Chance of
rain showers.
&&
.TIDES/COASTAL FLOODING...
* Minor Coastal Flooding, with significant erosion possible
The high tides of main concern are the early Wed morning and Wed
afternoon tide cycles. Thankfully, astronomical tides are quite
low which will limit the magnitude of flooding. A 3 to 3.5 ft
surge is possible with seas 20-25 ft offshore which will be
enough to produce areas of minor coastal flooding around the
times of high tide.
Another concern will be a persistent easterly wind wave over
multiple tide cycles. This may result in areas of significant
beach erosion, especially along the eastern Massachusetts shore.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through Wednesday evening for CTZ002>004.
Wind Advisory until 2 PM EDT Wednesday for CTZ004.
MA...High Wind Warning until 2 PM EDT Wednesday for MAZ007-013>018-
020-021.
Flood Watch through Wednesday evening for MAZ003>007-010>021-
026.
High Wind Warning until 5 PM EDT Wednesday for MAZ019-022>024.
Coastal Flood Advisory from 3 AM to 7 AM EDT Wednesday for
MAZ007-015-016-019-022>024.
Coastal Flood Advisory from 3 PM to 7 PM EDT Wednesday for
MAZ007-015-016-019-022>024.
Wind Advisory until 2 PM EDT Wednesday for MAZ005-006-012.
RI...High Wind Warning until 2 PM EDT Wednesday for RIZ005>008.
Flood Watch through Wednesday evening for RIZ001>008.
Wind Advisory until 2 PM EDT Wednesday for RIZ001>004.
MARINE...Storm Warning until 5 PM EDT Wednesday for ANZ232>235-256.
Storm Warning until 10 AM EDT Wednesday for ANZ230-236.
Storm Warning until 8 PM EDT Wednesday for ANZ231-250-254-255.
Storm Warning until 2 PM EDT Wednesday for ANZ237-251.
&&
SYNOPSIS...WTB/Loconto
NEAR TERM...WTB/Loconto
SHORT TERM...WTB
LONG TERM...Loconto
AVIATION...WTB/Loconto
MARINE...WTB/Loconto
TIDES/COASTAL FLOODING...